Many years ago, searching for the Internet over chess problem, I bumped into a very interesting piece.
Find it on the bottom as a FEN row, clicking on it the board will also appear.

The task is: white mates in one!

Can't be too hard, I thought, but scrolling over the possibilities the solution just didn't come up.
I check everything once more, then again, and again, and once I found another way that was hidden for me before. The solution also hid there. It became my favourite chess problem then, but of course, it's true, there are many many beautiful problems out there.